It has become common practice to provide implantable stimulating devices for stimulating the operation of a muscular organ, the operation of which has become impaired through injury to other portions of the body without injury to the organ itself. One example of this is the urinary bladder which is controlled by the brain through the spinal cord and a peripheral system of nerves connected between the spinal cord and the bladder. A disturbance to one of these nerves or nerve systems will often result in impairment of the ability of the patient to empty his bladder properly even though the muscle tissue of the bladder itself is healthy.
One method which has been employed very successfully in overcoming this problem is that of an implantable stimulator which is effective to stimulate the organ in question at desired times. Such a stimulator normally employs electrodes which are attached to the muscular organ at suitable points adjacent normal nerve connections and is effective to apply to such electrodes stimulus pulses at a frequency such that the organ is caused to operate in the desired manner. In the case of a urinary bladder, the stimulator is actuated at a time when it is desired to void the bladder.
While certain of such implantable units have a battery associated therewith, it is desirable where the implantable unit is to be operated only at preselected times to have the battery in an external transmitting unit so that the implantable unit employs purely passive components and receives all of its energy from the transmitting unit. In this way, it is possible to have an implantable unit which has an unlimited life.
Basically, there are two types of implantable units which receive their energy from an external source. One of these is the type which delivers the received energy directly, as with a tuned tank circuit, and transmits it in the form received to the electrodes. The other type is the type which has an energy storage element that is charged and periodically discharged to apply voltage pulses to the electrodes at the desired intervals.
The problem that has existed in connection with the latter type of unit is that the mechanism for controlling the discharge of the energy storage element has been relatively complicated. In one particular case, this has taken the form of a shift register which is advanced in accordance with pulses from the transmitting unit and which has variable outputs to selectively and periodically trigger electronic switches causing the energy storage devices associated with respective electrodes to discharge and apply a voltage to the associated electrodes.